The present invention relates to an adjusting dial of a thermal overload relay for adjusting a working current of the thermal overload relay, and, more specifically, the present invention relates to a structure of the relay which prevents an adjusting dial previously set from being mis-readjusted.
A thermal overload relay (hereinafter simply referred to as "relay") uses a bimetal plate for detecting an overload. The coupling length of a latch released by the thermal deformation of the bimetal plate is adjustable according to a kind of a load to facilitate adjustment of time from the start of flow of an overload current through the load until the disconnection of the load from its power supply. When the latch is released, a switch mechanism of the relay which has been in a waiting condition starts operating to switch on or off the relay contacts. As a result, an electromagnet of an electromagnetic contactor or an electromagnetic switch inserted in a main circuit in series with the relay is excited, and a main circuit current is interrupted. The coupling length of the latch is adjusted by an adjusting dial. Usually, a circular plate is fixed eccentrically to an end of a shaft of the adjusting dial. The coupling length of the circular plate and the latch placed at a predetermined position inside a relay case at a no-load condition is adjusted by rotating an angle of the adjusting dial.
FIGS. 8, 9(a) and 9(b) show a structure around the adjusting dial for preventing the adjusting dial from being mis-operated or mis-readjusted according to the prior art. In FIGS. 9(a) and 9(b), an adjusting dial 4 is positioned inside an adjusting dial mounting hole 4c of a case of a relay 1, and overload current values are graduated on the adjusting dial 4. A working current of the relay 1 determined by an overload current and a kind of the load is set by setting a corresponding mark of the overload current on the adjusting dial 4 to a dial setting mark 2b. The dial setting mark 2b is marked on an indication cover 2, on which a type of the relay, terminal symbols, etc. are stamped or printed. If the graduation setting is changed without notice, for example at a time of routine inspection, the relay may not operate or mis-operate when necessary and trouble may be caused in the equipment. In some relays which require high operation reliability, the adjusting dial 4 is covered by a dial cover 8 so that the adjusting dial can not be turned after setting. An example of the dial cover 8 is shown in FIG. 8, wherein the dial cover 8 with an engaging portion 8a is fitted into an adjusting dial insertion hole 2a of the indication cover 2 with a coupling protrusion 2c on the inner peripheral edge thereof.
In the above described prior art, the dial cover 8 may be easily removed with a usual tool as shown in FIG. 10 when a worker tries to operate the adjusting dial by misunderstanding.
To meet the requirement that the adjusting dial 4 previously set will never be operated again, following measures have been taken.
(1) The operating hole 4a of the adjusting dial 4 is filled with resin, and the adjusting dial 4 is fixed to the case with the resin.
(2) The dial cover 8 is bonded to the indication cover 2 in a manner that the dial cover 8 will never be removed.
However, as the resin, two-liquid resin is often used in the above case, wherein before using the two-liquid resin, two kinds of resin liquids have to be mixed. Thus, there are following drawbacks.
(a) It takes time to mix two kinds of resin liquids.
(b) It takes about a day before the filled resin completely hardens.
(c) Since the resin once mixed hardens naturally, the mixed resin can not be stored.
These drawbacks cause labor to satisfy the requirements to thereby increase cost. Moreover, it can not meet a requirement of quick installment of the relay.
In view of the foregoing, an object of the present invention is to provide a thermal overload relay having a structure which facilitates removal of a dial cover when necessity of readjustment of an adjusting dial of the relay is confirmed, and which prevents the dial cover from being removed when the adjusting dial shall never be readjusted.